Crystal-controlled oscillation generator and associated circuitry



Nov. 30, 1965 c. G.

CRYSTAL-CONTROLLED OSCILLATION GENERATOR DE BLASIO AND ASSOCIATED CIRCUITRY 1955 2 Sheets-Sheet 1 Original Filed Nov. 28,

INVENTOR Conrad G. Dz Haste wwm w A OR BY Nov. 30,

FIQ.5.

, C. G. DE BLASIO CRYSTAL-CONTROLLED OSCILLATION GENERATOR AND ASSOCIATED CIRCUITRY Onginal Flled Nov. 28, 1955 2 Sheets-Sheet 2 F equency deviofion diagram For one read'once rube,,|8or|9 grid bias nonlinear region linear region Frequzncy operaiing, or quiescn+ pain range of opera'h' Frequency dzv'lc'l'ion diagram For borh reaoi'ance 1ubes,l8and l9 non-Imamregion linear region TUBE l8 IN PUT MODULATION SIGNALS f rz quency f erues-resonpnl' freq.) "near 3' or zero slgnal nonlinear region INVENTOR Conrad Dz Blasio United States Patent 3,221,268 CRYSTAL-CGNTROLLED OSCILLATION GENER- ATOR AND ASSGCIATED CIRCUITRY Conrad G. De Blasio, West Long Branch, NJ. Electronic Measurements Co., Lewis St. and Maple Ave.,

Eatontown, NJ.)

Original application Nov. 28, 1955, Ser. No. 549,435, new Patent N0. 2,962,672, dated Nov. 29, 1960. Divided and this application Nov. 23, 1960, Scr. No. 71,197

Claims. (Cl. 33199) My invention relates to improvements in crystal-controlled oscillation generators and associated circuitry for developing a carrier wave at a given frequency; and particularly to such a generator and associated circuitry for special application in a dual-tube, balanced-reactance modulator circuit or system such as is disclosed in Letters Patent No. 2,962,672 issued to me November 29, 1960, and of which the subject matter claimed herein is a division.

For the purpose of illustrating my invention an embodiment thereof is shown in the drawings, wherein FIGS. 1 and 2 are simplified, diagrammatic views illustrative of the method or manner, in my improved circuit, of avoiding or eliminating parallel resonance attributable to capacitance between the holder and contact plates of the crystal;

FIG. 3 is a diagrammatic view of a crystal controlled oscillation generator and associated circuitry embodying my invention and incorporated in the frequency modulated oscillator circuit disclosed and claimed in the aforesaid patent; and

FIGS. 4 and 5 are simplified, graphical presentations illustrative of the overall frequency-shift performance in a dual-tnbe modulator employing my improved crystal controlled Oscillation generator and its associated circuitry.

In the various circuits proposed heretofore to employ a piezoelectric crystal for the frequency-determining element in an oscillator, it is not possible to obtain appreciable amounts of linear, controlled shift. The reason for this insufficiency is attributable to the crystal serving as the resonant element in such oscillators. This crystal is represented by the electrical circuit in FIG. 1 wherein C and L are the equivalent electrical constants of the crystal. C is the holder and contact plate capacitance. The terminal impedance of the network shown in FIG. 1 can be expressed as This circuit or network has two resonance frequencies, i.e., a series resonance determined by C and L and a parallel resonance when it is considered that C is part of the resonant circuit. Keeping in mind the fact that the ratio of C to C is normally of the order of 1000 or more, it will be observed from the above expression that the two resonant modes lie very close to each other in frequency. This expression also reveals that a variation of the shunt element C will have no effect on the series resonance, and only slight effect on the parallel resonance.

In my improved system or circuit, for the purpose of providing access to the series-resonance elements C and L a lumped-constant transmission line is added to the network, as shown in FIG. 2. This line is designed to have an electrical wavelength of a quarter wave at the operating frequency. C is made at least equal to C for the purpose of lumping the latter with C thereby causing C to vanish. Also, under the same conditions the series-resonance impedance of C L will appear at terminals AB as a high impedance because of the well known "ice properties of a quarter-wave line. Furthermore, any re actance connected to the output terminals AB will appear as a reactance of opposite sign in the crystal-resonance circuit. By this expedient, connection at AB is a direct one, to the series-resonance circuit. This makes possible a controlled deviation or alteration of the series-resonance frequency, and consequently a corresponding change or variation in the maximum impedance frequency of the total network measured across terminals AB. As this network is shown in FIG. 2, however, there is the undesirable performance characteristic which resides in the fact that at frequencies greatly removed from the initial resonance frequency of the crystal, parasitic resonances may be derived to produce a more favorable impedance at AB than for the useful frequencies. In an oscillator circuit this would lead to frequency jumping and to a loss of crystal control when operating over a broad frequency range. This tendency could be reduced by introducing elements across the crystal circuit. However, this would impair, somewhat, action of the crystal. Another difiiculty or problem resides in the fact that any reactance inserted in the crystal-resonance circuit is but a small percentage of either X or X since the ratio of L to C is very high. Accordingly, much larger reactance sweeps are necessary to produce a given change in frequency than would be necessary in a conventional, parallelresonant tuned circuit. It is difficult to produce such larger reactance sweeps linearly, by electronic means.

It is with the foregoing in mind that reference is now made to FIG. 3 disclosing a crystal controlled oscillation generator and associated circuitry for developing a carrier wave at a given frequency and embodying my invention; this generator and associated circuitry being shown ap plied, in a novel way, in a dual-tube, balanced-reactance modulator circuit or system. The combination and joint operation or cooperative action of the improved oscillation generator and associated circuitry and the novel dualtube modulator also are important aspects of my invention. For convenient identification and purposes of terminology, the circuit in FIG. 3 is broken down into three distinct sections or parts bracketed, respectively, as a crystal-controlled oscillation generator and associated circuitry 10, a modulator 11, and frequency-deviation or range-changing means 12.

The oscillator section 10 embodies a frequency-determining element in the form of a piezoelectric crystal 15 having conventional terminals for connecting the crystal in the circuit shown, and an oscillator tube 16 having electrodes including a grid 16a, an anode 1612, a cathode 16c, and a screen grid 16d. Interposed between crystal 15 and tube 16 are the variable inductive reactance L and the variable capacitive reactance or capacitor C, as in FIG. 2, to constitute a quarter-wave line which eliminates or cancels out the capacitance C which otherwise would be present, as indicated in dashline. The crystal oscillator section 10 is unique in the sense that the lumpedconstant transmission line comprised of L C and C gains direct connection to the series-resonant elements C and L of crystal 15. The crystal-holder capacitance C becomes part of the quarter-wave line. The capacitance C may constitute all or some part of the line input capacitance. The inductor L is adjusted to produce the quarter-wave condition at the carrier frequency. Oscillation is maintained by tube 16; and a coupling, passband transformer T is adjusted, by a movable core represented at t, to attenuate modes of operation lying outside the useful range of operating frequencies, without resorting to loading of crystal 15. The circuit elements shown associated with oscillator tube 16 are arranged, chosen or adjusted so that oscillation occurs at a very low level to reduce thereby the previous, required magnitude of reactive modulating currents which are difiicult to produce and which might, otherwise, unduly load crystal 15 or the oscillator circuit. The circuit elements or means for this purpose include a variable capacitance 17 connected, as shown in FIG. 3, across the cathode 16c and the screen grid 16d of tube 16, the capacitance 17 being functional at the correct adjustment of the same to effect oscillation of tube 16 at the desired, relatively low level.

Transformer T is functional in several respects. The winding 1 thereof is a relatively high impedance inductor of low distributed capacitance which couples into the quarter-wave system without disturbance. The transformer winding 2 is a tuned winding which, together with winding 1, provides a broadly tuned passband system which inhibits spurious modes of oscillation which otherwise would severely restrict operating range. The transformer T, provides phase-inversion and impedance matching as required by oscillator tube 16. The turns-ratio-adjustment promotes low-level operation which allows the modulator to work more easily. Still another function of transformer T resides in the fact that it permits use of a pickoif system comprising capacitors a and c, and resistor e. Such a system needs no buffer, and does not disturb the oscillator 16.

The modulator section 11 comprises or includes a pair of modulator pentode tubes 18 and 19 whose combined effects or respective operating actions add up to a balanced-reactance modulator which provides electronic sweep of frequency by a virtual change of reactance, and which achieves phase inversion and balanced modulation without resorting to electron tubes other than the two modulator tubes 18 and 19. Resistor 64 is made relatively large and aids balance, produced by the resistors hereinafter referred to, in an analogous manner.

Voltage feed for the tube or stage 18 is arranged to provide quadrature current in the anode circuit 18a in proper direction to effect a capacitive reactance. To this end the grid voltage at tube 16 is, through a coupling capacitor 20, a capacitor 21, and a coupling capacitor 22 applied to grid 18b so that the resultant voltage at 18b leads the original voltage at 23 by ninety degrees. Thus, the plate current of tube 18 produces a capacitive reactance across capacitor C Voltage feed for the tube or stage 19 is arranged to provide quadrature current in the anode circuit 19a in proper direction to effect an inductive reactance. To this end the grid voltage at tube 16 is, through a coupling capacitor 25, a resistor 26, an inductive reactance 27, and capacitor 22 applied to grid 1% so that the resultant voltage at 19b lags the original voltage at 23 by ninety degrees. Thus, the plate current of tube 19 produces an inductive reactance across capacitor C Input modulation signals are applied to grids 18b and 19b through suitable filters 28 and 29, respectively. Each of the cathode by-pass capacitors 30.and 31 is effective for radio frequencies, but each provides a high impedance for DC. or audio frequencies. The cathodes 18c and 190 can therefore be considered as being coupled together for modulating currents. The capacitors 30 and 31, therefore, bypass the respective cathodes 18c and 190 for radio frequencies but the latter are coupled for audio or modulating frequencies by resistance means in the form of a cathode-coupling network or coupling impedance comprised of resistors 32, 33 and 34 connected between cathodes 18c and 190 and series-connected with respect to each other. As shown in FIG. 3, in the coupling network for audio or modulating frequencies the resistors 32 and 34 thereof are individual respectively to the cathodes 18c and 190, and the variable or adjustable resistor or potentiometer 33 provided with the conventional slider or equivalent moveable element shown, is connected between resistors 32 and 34. To the slider or adjustable element of resisfor 33 there is connected one end of a very large resistor 35, the other end of the latter being connected as indicated, to a suitable source of negative potential Ec which is made largefor thereason hereinafter'explained.

The resistive cathode impedance of an un-bypassed stage is given by the approximate expression r =l/gm. Since the coupling impedance is very large it can be ignored, which then gives a cathode of impedance l/gm driving another cathode of impedance l/ gm. From this it will be seen that half the signal voltage will appear at each of the two cathodes 18c and 190, and inversion with respect to modulating frequencies will result. By way of the resistor 35 there is provided the negative potential Ec which is made large so that this resistor can be very large whilst proper operating currents are maintained for tubes 18 and 19. A corollary or resulting advantage is that the total current becomes substantially independent of tube characteristics since it will always remain in the vicinity of Ec/R The system can be balanced by adjusting resistor 33.

Applying a plus voltage at input terminal 36v increases the grid voltage of tube 18, increasing the cathode. voltage of this tube. Since the cathodes 18c and are coupled as explained, the cathode voltage of tube 19 also increases to increase the grid-cathode bias of tube 19. Because of a change in transconductance, the net result is a decrease of radiofrequency plate current of tube 19 and an increase of radiofrequency plate current of tube 18. In other words, tube 18 draws more reactive plate current with a plus input on terminal 36, causing tube 19 to draw less reactive plate current due to the, cathode coupling. There is then an unbalance in the reactance, producing at least twice the swing or deviation, as graphically shown by FIGS. 4 and 5, than could be achieved if only one tube were used.

When a minus voltage is impressed at 36, tube 18 draws less current causing tube 19 to draw more current, because of the common resistive coupling. Similar operation takes place with plus and minus voltages at the input terminal 37.

In FIG. 4, which is a plot for a single reactance tube, i.e., 18 or 19, the range of bias gives the indicated deviation. From FIG. 5 it will be seen that by using the two tubes 18 and 19 connected for opposing reactance at RF frequencies but coupled for audio or modulating frequencies, there is at least twice the swing for the same input, and at least twice the range is obtained, with equal linearity. This graph also shows that the modulating system or section 11 is balanced and that at the series-resonant frequency, i.e., with zero signal at 36, 37; the reactances of tubes 18 and 19 cancel each other. Also shown is the fact that larger modulations are possible since with a given direction of modulation the reactance of one of the tubes 18 and 19 increases and that of the other decreases. Either direction of modulation is available by selection of one of the two grids 18b and 19b. More linear modulation results both from the increased range and from the compensating effect of the two reactance tubes 18 and 19. Large cathode degeneration provides for good stability.

When a frequency-modulation oscillator system, in the same general class as that shown in FIG. 3, is used for radio-frequency transmission purposes it becomes necessary to divide the modulation by integers to compensate for multiplication in following radio amplifier stages. The novel arrangement and circuit of section 12 serves this purpose and has the advantage of providing an autotransformer effect for modulating reactance currents, using simple resistive elements. The anodes 18d and 19d of reactance tubes 18 and 19, which are tied together by connection 38, are tapped down on a bleeder network comprising resistive elements 39, 40, 41 and 42 which, by rotation of switch 43 can be connected in various combinations across the frequency-determining network. The reactance stages 18 and 19 are pentodes. As is well known, the anode current of a pentode is substantially unaffected by load impedance and is given approximately by g e where :2 is the exciting voltage for each grid. If the junction currents at the switch contact 47 are added, it is clear that n= 1+ 2; A150, rflRssrl-RT) 2( 4o 41,+ 42) where R is the impedance of the frequency-determining network. In other words, the available (invariant) modulation current can be divided into a useful component, i and a discarded component, i The proportion is,

In accordance with this proportion taps are added to the bleeder to provide the degrees of shift required.

The above simple relationship ignores reactances appearing at the reactance tube anodes 18d and 19d. Such reactances are canceled by the elements 44, 45 and 46 which together constitute a resonant circuit at the frequency of operation, and act to absorb any reactance on line 48. Thereafter, only a lossy component is effective on line 48.

It is important to adjust division precisely. This cannot be done on the autobleeder alone, because the adjustable elements would be excessively reactive. The desired result is accomplished by splitting the common coupling impedance for cathodes 18c and 190 into the resistive elements 32 and 34. A correct degree of audio degeneration is obtained by rotation of switch 49 to connect one of the trimmer resistors 50, 51 and 52 across the coupling impedance, or to short the latter. Such degeneration is not in effect for radio frequencies.

By means of a switch 53 the capacitors 54, 55, 56 and 57 are properly connected in the circuit, with respect to resistors 39, 40, 41 and 42 to permit elimination of small zero shifts caused by switching.

The elements 44, 45 and 46 are chosen or adjusted to be resonant at the carrier frequency, usually being very close or equal to the series-resonant frequency of crystal 15. No further change or adjustment in these elements need be made during operation.

Switches 43, 49 and 53 are geared or otherwise ganged, as indicated, for simultaneous rotation thereof by a common, single knob represented at 58. For any adjustment or sweep change made by turning knob 58, the movement of each of the three switches 43, 49, and 53 is the same in direction, degrees, and orientation. For example, and as shown with switch 43 on contact 47 thereof, switches 49 and 53 are on the contacts 59 and 60, respectively. Similarly, with switch 43 on the contact 61 thereof, switches 49 and 53 are on the contacts 62 and 63, respectively. In each of the other positions of knob 58 the three switches 43, 49 and 53 are in the respective positions thereof for correct coordination of resistors 39, 40, 41 and 42; trimmers 50, 51 and 52; and capacitors 54, 55, 56 and 57.

From the foregoing it will be seen that by turning the single knob 58, different portions of the output or plate currents of reactance tubes 18 and 19 are tapped off. The values of resistors 39, 40, 41 and 42 are related so that in the different positions of switch 43 the reactance swing is divided by integral multiples. These multiples are adjusted exactly by the Vernier or trimmer resistors 50, 51 and 52 which adjust degeneration, and effect an increase in the cathode coupling between tubes 18 and 19, thus changing the magnitude of the reactance swing. That is, trimmers 50, 51 and 52 are used to adjust the modulation range to the exact multiple by adjusting degeneration at audio frequencies for the balanced modulators 18 and 19. In other words, operation of the frequency-dividing system or section 12 is accomplished simply by turning the single knob 58 which causes, simultaneously, rotary movement of switch 43 to effect a change in the tap on the autobleeder, rotary movement of switch 49 to effect a change of division trimmer, and rotary movement of switch 53 to effect a change in zero adjustment to offset slight errors in frequency resulting from switching.

In the operation of my improved system or circuit, a modulating voltage is applied to either input, tube 18 being for capacitive reactance and tube 19 being for inductive reactance. This results in an unbalance of the modulator 6 plate currents so that either a lagging or a leading current is impressed on the oscillator circuit or section 10. The reactive current then produces a net change in the seriesresonant frequency of the crystal oscillator circuit. When radio amplifier stages follow the frequency modulated oscillator, as in the case where the FM oscillator system herein is used for radiofrequency transmission purposes, knob 58 of section 12 is adjusted to divide the modulation by integers, thereby compensating for multiplication in such radio amplifier stages.

One of the broader aspects of my invention as claimed herein is considered to reside in the aforesaid improvements embodied in the disclosed crystal-controlled oscillation generator 10 and associated circuitry whereby there is avoided or eliminated parallel resonance which otherwise would be present and attributable to the capacitance C between the holder and contact plates of the crystal; the generator 10 comprising a resonant or frequency-determining element in the form of the piezoelectric crystal 15 having a given operating or carrier frequency and the novel crystal-resonance circuit or network shown, the oscillator tube 16 having its grid-cathode circuit (16a, connected across the output terminals AB of said network and functional to maintain oscillation of the generator 10, the lumped-constant transmission line connected in and forming part of said network and with the crystal-holder capacitance C effective as part thereof having an electrical wavelength of a quarter wave at said carrier frequency, said network with said quarter-wave line including the crystal-holder capacitance C as aforesaid establishing the operating condition of the series-resonance impedance of the crystal and its associated circuitry appearing at the output terminals AB as a relatively high impedance and establishing also the operating condition residing in the fact that reactance connected to the output terminals AB appears as a reactance of opposite sign in the crystal-resonance circuit, the generator by virtue of the aforesaid operating conditions being characterized by the fact that connection across the output terminals AB of said network is a direct connection to the series-resonant elements of the crystal, and means such as the transformer T associated with and connected with respect to said network and to the electrodes of the oscillator tube 16 as shown in FIG. 3 and functional to promote relatively low-level operation of the generator 10.

With regard to my invention as claimed herein, reference is made to the piezoelectric crystal circuit arrangement disclosed in Patent No. 2,551,809 issued May 8, 1951 to Wilfrid S. Mortley.

It will be understood that in my improved system various modifications such as in circuit and structural arrangements, are possible and would be within the conception of those skilled in the art without departing from the spirit of my invention or the scope of the claims.

I claim as my invention:

1. In a crystal controlled oscillation generator and as sociated circuitry of the character described for developing a carrier wave at a given frequency; an oscillator tube having a grid and an anode, a piezoelectric crystal having holder and contact plates, an inductive reactance having two terminals connected respectively to said grid and to one of said plates, a capacitive reactance having two terminals connected respectively to said grid and to the other of said plates, said inductive reactance and said capacitive reactance as connected aforesaid constituting jointly with the intrinsic crystal-holder capacitance between said plates a quarter-wave transmission line substantially at said frequency, and a pass-band transformer having inductively-coupled windings, one of said windings at an end thereof being connected to a terimnal of each of said reactances, the other of said windings at an end thereof being connected to said anode, said transformer as connected aforesaid being functional to attenuate modes of operation lying outside the useful range of operating frequencies.

2. In a crystal controlled oscillation generator and associated circuitry of the character described for developing a carrier wave at a given frequency; an oscillator tube having a grid and an anode, a piezoelectric crystal having holder and contact plates, an inductive reactance having two terminals connected respectively to said grid and to one of said plates, a capacitive reactance having two terminals connected respectively to said grid and to the other of said plates, said inductive reactance and said capacitive reactance as connected aforesaid constituting jointly with the intrinsic crystal-holder capacitance between said plates a quarter-wave transmission line substantially at said frequency, and a pass-band transformer having a first winding constituting a relatively high impedance inductor of relatively low distributed capacity coupled into said quarter-Wave line and having a second winding inductively coupled with respect to said first Winding, said first winding at an end thereof being connected to a terminal of each of said reactances, the other of said windings at an end thereof being connected to said anode, said transformer connected as aforesaid being functional to provide for phase-inversion and impedance matching as required by said oscillator tube.

3. In a crystal controlled oscillation generator and associated circuitry of the character described for developing a carrier wave at a given frequency; an oscillator tube having a cathode and a grid and an anode, a piezoelectric crystal having two terminals, an inductive reactance and a capacitive reactance each having two terminals, a terminal of said inductive reactance and a terminal of said capacitive reactance being connected to each other and to said grid, the other terminal of said inductive reactance being connected to a terminal of said crystal, the other terminal of said capacitive reactance being connected to the other terminal of said crystal and to said cathode, said inductive reactance and said capacitive reactance connected as aforesaid constituting jointly with the intrinsic crystal-holder capacitance a quarter-wave transmission line substantially at said frequency, and means connected across said grid and said anode for attenuating modes of operation lying outside the useful range of operating frequencies of said generator.

4. In a crystal controlled oscillation generator and associated circuitry of the character described for developing a carrier wave at a given frequency; an oscillator tube having a cathode and a grid and a screen grid, a piezoelectric crystal having two terminals, an inductive reactance and a capacitive reactance each having two terminals, a terminal of said inductive reactance and a terminal of said capacitive reactance being connected to each other and to said grid, the other terminal of said inductive reactance being connected to a terminal of said crystal, the other terminal of said capacitive reactance being connected to the other terminal of said crystal and to said cathode, said inductive reactance and said capacitive reactance connected as aforesaid constituting jointly with the intrinsic crystal-holder capacitance a quarter-Wave transmission line substantially at said frequency, and a variable capacitance having two terminals one of which being connected to said cathode, the other terminal of said variable capacitance being connected to said screen grid, said variable capacitance being adjusted to effect oscillation of said tube at a relatively low level.

5. In a crystal controlled oscillation generator of the character described; a frequency-determining element in the form of a piezoelectric crystal having two terminals and a given carrier frequency, a network connected across the terminals of said crystal and constituting therefor a crystal-resonance circuit, output terminals for said network, an oscillator tube having a grid-cathode circuit connected across said output terminals and being functional to maintain oscillation of said generator, a lumped-constant transmission line connected in and forming part of said network and with the crystal-holder capacitance effective as part thereof having an electrical wavelength of a quarter wave at said carrier frequency, said network with said quarter-wave line including the crystal-holder capacitance as aforesaid establishing the operating condition of the series-resonance impedance of said crystal and its associated circuitry appearing at said output terminals as a relatively high impedance and establishing alsothe operating condition residing in the fact that reactance connected to said output terminals appears as a reactance of opposite sign in said crystal-resonance circuit, said generator by virtue of said operating conditions being characterized by the fact that connection across said output terminals is a direct connection to the series-resonant elements of said crystal, and a coupling transformer comprising a first winding in the form of a relatively high impedance inductor of relatively low distributed capacitance connected at an end thereof to said network and a second winding tuned to provide with said first winding a broadly tuned passband system, said transformer providing phase-inversion and impedance matching as required by said tube.

References Citedby the Examiner UNITED STATES PATENTS 2,600,124 6/1952 Mortley 332-26 ROY LAKE, Primary Examiner.

JOHN KOMINSKI, Examiner. 

1. IN A CRYSTAL CONTROLLED OSCILLATION GENERATOR AND ASSOCIATED CIRCUITRY OF THE CHARACTER DESCRIBED FOR DEVELOPING A CARRIER WAVE AT A GIVEN FREQUUENCY; AN OSCILLATOR TUBE HAVING A GRID AND AN ANODE, A PIEZOELECTRIC CRYSTAL HAVING HOLDER AND CONTACT PLATES, AN INDUCTIVE REACTANCE HAVING TWO TERMINALS CONNECTED RESPECTIVELY TO SAID GRID AND TO ONE OF SAID PLATES, A CAPACITIVE REACTANCE HAVING TWO TERMINALS CONNECTED RESPECTIVELY TO SAID GRID AND TO THE OTHER OF SAID PLATES, SAID INDUCTIVE REACTANCE AND SAID CAPACITIVE REACTANCE AS CONNECTED AFORESAID CONSTITUTING JOINTLY WITH THE INTRINSIC CRYSTAL-HOLDER CAPACITANCE BETWEEN SAID PLATES A QUARTER-WAVE TRANSMISSION LINE SUBSTANTIALLY AT SAID FREQUENCY, AND A PASS-BAND TRANSFORMER HAVING INDUCTIVELY-COUPLED WINDINGS, ONE OF SAID WINDINGS AT AN END THEREOF BEING COINNECTED TO A TERMINAL OF EACH OF SAID REACTANCES, THE OTHER OF SAID WINDINGS AT AN END THEREOF BEING CONNECTED TO SAID ANODE, SAID TRANSFORMER AS CONNECTED AFORESAID BEING FUNCTIONAL TO ATTENUATE MODES OF OPERATION LYING OUTSIDE THE USEFUL RANGE OF OPERATING FREQUENCIES. 